Could you
expand on the design and construction flexibility of reinforced concrete columns?
Columns can be optimized for size, shape and cost. Cost
optimization can be achieved by varying the column size, shape, the concrete strength, and
the rebar yield strength and percentage of reinforcement. Often, changes can be made on
short notice. Concrete strengths greater than 14,000 psi [98 MPa] have been used in column
construction. Various combinations of concrete strengths and percentage of reinforcement
should be evaluated to obtain the optimum cost for each specific project.
Fig. 1 Cost Comparisons for round and square columns. (Basis: "Cost of
Reinforced Concrete High-Rise Columns, "Structural Bulletin No. 11, CRSI)
If a column is built out-of-alignment,
what can be done?
Reinforced concrete columns and frames offer an
inherent tolerance to compensate for a reasonable amount of eccentricity. Out-of-alignment
columns create a moment due to the eccentricity. This moment must be compensated by a
force couple in the floor system. In most cases reinforced concrete floors have sufficient
strength to resist these axial forces without added reinforcement. Additional loadings
must always be checked by the architect/engineer.
Is it true that circular columns
require additional furring to fit the wall configuration?
There is a perception that circular columns require additional furring
to support drywall. Most contractors prefer to frame walls to compensate for vertical
alignment within allowable tolerances. Furring costs are generally the same for round and
square columns.
Lap splices for larger reinforcing
bars are long and make construction more difficult. And lap splices for #14 and #18 bars
are not permitted except to smaller size bars in compression. Any suggestions?
Mechanical splices of different types are readily available for
splicing rebar. For compression-only situations, end-bearing mechanical splices are a
cost-effective option. Refer to CRSI's newly revised Reinforcement Anchorages and
Splices book for further information.
Are round-tied columns less expensive?
Figure I illustrates the impact of column shapes
carrying the same axial load for a specific project. Detailed evaluations of various
combinations of size, shape, material strength, and percent of reinforcement should be
conducted to find the optimal design for the architect/engineer and the owner.
Are round columns less expensive than
other shapes, and how is an optimum column design reached?
In general, round columns are less expensive than other shapes. A major
cost factor is the availability of prefabricated forms. Prefabricated forms for standard
size round columns are readily available. Even unusually-shaped columns have been
constructed economically if the formwork is available prior to construction. Stripping and
re-erection of circular forms for round columns is simple and quick.
Prefabricated forms can shape irregular columns economically.
ENGINEERING
INFORMATION PROGRAM-DECEMBER 1997 |
